someone on reddit estimated it's about 1900 miles based on this world map (I didn't look closely enough at it to see if there were spoilers in the image)
http://www.sermountaingoat.co.uk/map...canon_only.jpg
then there's been discussions about maximum raven/dragon speed which could be anywhere from 25 to 50 mph. basically it's something that would have taken closer to a week than a day, but as with every other journey this season, they gotta wrap this sucker up. I bet Euron could have made the round trip in like a lunch hour.
just try to pretend like Westeros is no longer the size of South America as originally designed, and is instead more like the British Isles and it won't feel so weird.